Well, I really think you can look at this several ways. You can just “wing it” and see what happens. You can put some thought into it, and maybe that’s better than nothing. Or you could take it pretty seriously and give it a fair amount of thought and put a plan into place. I’ve had drive failures without backups, and even though the data wasn’t critical it was incredibly annoying.
So, think about what data you put where, and then consider a scenario where something fails.
Well, but you can’t back up your larger drive to your smaller drive. If you use 300GB of storage on your smaller system drive then you will only have 200GB of the 1TB backed up on it. And if you do that the system drive is full, might slow down, and you certainly have no space to install more software. So that’s a problem.
And vice versa you can obviously back up your system drive onto your larger drive. But the way it stands now you’re eating up potentially half that space. That could be fine of course, and you’re also likely to not use all 500GB of the system drive so you’re going to lose less than that one the larger one. But still, you don’t want to be in a situation where you add some software an all of a sudden your system drive takes up 420GB, but you only have 350 free on the other one… IF you want to back the system drive up that is.
Now also imagine that you end up with a drive failure. Your system drive fails. If you have “music data” (project files, audio files, samples etc) on that drive then not only did you just lose your OS and your system won’t boot, but you might not even be able to take that drive out and access your music data. So you lost “twice” as much as if you had things separately.
So yes, I would recommend keeping software and “data” separate.
I actually just checked again and on Amazon (US) I see an external USB drive with a 2TB capacity for $60. Backing up will initially be slow, but you’ll be safe.